Page:VCH Cornwall 1.djvu/85

 GEOLOGY excepting the narrow strip which forms its eastern margin, where be- tween Mevagissey Bay and Gerrans Bay the grey quartzites yielded to the late Mr. C. W. Peach fossils of Caradoc or Bala age. In the south- westerly prolongation of that zone, which traverses the Meneage peninsula and contains similar lithological types, the quartzites have yielded fossils to Mr. J. H. Collins, while the limestones of both areas enclose frag- mental remains of crinoids. It will be seen therefore that the succession from the Culm Measures (Carboniferous) to the Caradoc (Ordovician) is broken by the geological chapters which separate the latter epoch from the Lower Devonian. The relations which the barren deposits of south-west Cornwall bear to those missing chapters have not been determined. Further, we are not aware of the exact base of the Devonian in Cornwall. Even within the present year Mr. Clement Reid in his examination of the north coast in the vicinity of Newquay has extended the palasontological hori- zons to lower limits of the Devonian system than hitherto discovered in that locality. In view of the uncertainties which attend the unravelling of the Palaeozoic sequence we have not placed the inquiry in a prominent position, and more especially because the subsequent history of those for- mations in the subterranean depths has impressed upon our county its dominant geological type. It is therefore on this stage in the building up of Cornwall that we have more particularly dwelt. In recent years an important advance has been made in the strati- graphy of west Cornwall by Mr. Howard Fox's discovery of radiolarian cherts at Mullion Island in association with lavas of a peculiar pillowy or sacklike appearance, an association closely resembling that found to occur in the Arenig beds of the south of Scotland. Moreover Mr. Fox has traced these radiolarian horizons into the Gorran area. Further, in the latter district we have found the pillow lavas of Mullion Island on a similar geological horizon associated with limestone and quartzite; so that the Ordovician age of the Cornish pillow lavas and their associated cherts has been clearly established. While in the Veryan area these Ordovician beds appear to pass quite conformably into the barren deposits which extend so largely over the south-west of Cornwall, in the Meneage peninsula, their continuity appears to be broken by a conglomerate. That coarse deposit includes pebbles of distantly derived material, which makes it a very impor- tant horizon. For not only amongst its contents do we find boulders of quartzite, but amongst its finer detritus the microscope reveals the pres- ence of the pillow lavas. The conglomerate also contains foliated igneous rocks, some of which have certainly been derived from the Lizard area, so that we have in Cornwall at least two periods of meta- morphism, one of which is prior to the formation of the conglomerate and its included pebbles of pillow lava, and one later. Of the age of the former we have no evidence, except that it must be pre-Arenig, while the latter is posterior to the Culm Measures. 37